They cry a lot and are expensive.
They also go poo poo quite often. Gross.
In some countries they are used as sex slaves.
None of this matters as I am sure Obsidian wants to be able to sell the game and killable children tends to put a crimp in sales for some reason or other.
If Obsidian's end goal was money, Project Eternity wouldn't exist. If Obsidian wanted money, they would make another AAA game for a publisher that would be targeted at a much broader consumer base. PE is not going to make Obsidian rich. PE is all about nostalgia and the hard core, old school RPG fan base. I don't think that group will mind if pixel children can be killed. If the game is banned in Germany, so what? There are alternatives for Germans to get the game.
On a side note, when I suggested children as potential companions, I did not have toddlers in mind . Think of this potential companion as an apprentice, maybe 13 or 14 years old, not initially effective in combat but on a steep learning curve.